The present invention relates to positioning control method and apparatus of a head which is used in a magnetic disk apparatus for positioning the head to the designated cylinder position and, more particularly, to positioning control method and apparatus of a head for positioning the head by correcting characteristics of a head actuator so as to eliminate a variation of the characteristics of the head actuator which differ in dependence on the rotating position of a magnetic disk.
In recent years, in a magnetic disk apparatus, a high accessing speed and a high positioning accuracy are needed in association of the realization of a high operating speed and a large capacity. When such a high accurate positioning control is performed, a variation in mechanical accuracy of the head actuator, a gain error of an electric servo loop, and the like become obstacles.
In the conventional magnetic disk apparatus, a head 12 is positioned by using a voice coil motor (VCM) 10 as shown in FIG. 1 . In FIG. 1, the voice coil motor 10 has a structure such that a force is generated by both of fixed permanent magnets 24 and movable coils 26. Now, assuming that a magnetic flux density which is applied from the permanent magnet 24 to the coil 26 is set to B and an effective length of the coil is set to L and a current flowing in the coil is set to I, a force F is obtained by the following equation. EQU F=B.times.L.times.I (1)
Therefore, the force F which acts on the voice coil motor 10 and which is expressed by the equation (1) can be controlled by the current I flowing in the coil 26 and the head 12 can be positioned to a target cylinder position over a magnetic disc 22.
In such a conventional voice coil motor 10, however, since the permanent magnet 24 has a finite length, there are characteristics such that the magnetic flux density B is small in the edge portion of the permanent magnet 24 and is large in the central portion. The force F which is applied to the voice coil motor 10 consequently fluctuates due to a variation in magnetic flux density B in dependence on the position of the permanent magnet 24 and a positioning control accuracy is not improved.
On the other hand, a variation in magnetic flux density B of each magnet is large on manufacturing. The force F is influenced by a variation in permanent magnets 24 assembled in the apparatus, a variation in constants of an electric circuit in a control loop, or the like, so that the high accurate positioning control is obstructed.
FIG. 2 shows a change in force (relative value) when the head position is changed every other predetermined number of cylinders with respect to voice coil motors used in three magnetic disk apparatuses as an example. For instance, as will be obviously understood from characteristics 28, the relative value of the force shown by an axis of ordinate decreases in association with the movement of the head from the center position to the inner side and there is also a decreasing tendency of such a force with respect to the head movement toward the outer side.
With respect to the other voice coil motors, there are variations as shown in characteristics 30 and 32 and a change in force to a change in head position has a peculiar value although they have similar tendencies. It is, consequently, difficult to perform the positioning control of a high accuracy due to a variation in characteristics of the voice coil motors.